The present invention relates generally to improved materials and methods for pattern formation on semiconductor wafers, and more particularly, to improved photoresist materials for use in lithography.
Photolithography employs photoresists, which are photosensitive films, for transfer of images, e.g., negative and positive images, onto a substrate, e.g., a semiconductor wafer. Subsequent to coating a substrate with a photoresist, the coated substrate is exposed to a source of activating radiation, which causes a chemical transformation in the exposed areas of the surface. The photo-resist coated substrate is then treated with a developer solution to dissolve or otherwise remove either the radiation-exposed or unexposed areas of the coated substrate, depending on the type of photoresist employed.
A wide variety of energy sources, such as X-rays, extreme ultra violet (EUV), low and high kV electrons, ion beams, and extended optical wavelengths, e.g., 248, 193, and 157 nm radiation, can potentially be employed for advanced sub-100 nm imaging. Conventional microlithography techniques for creation of features having sizes of 100 nanometers or less, however, suffer from a number of shortcomings. For example, linewidth variations of a resist film produced by such techniques can be too large to be acceptable in view of high dimensional tolerances typically required in this range, e.g., tolerances of the order of the scales of the molecular components of the resist film. Such linewidth variations are referred to as line edge roughness (LER).
Line edge roughness (LER) causes linewidth fluctuations that may lead to variations in device characteristics. As critical dimensions for integrated circuits continue to shrink, linewidth fluctuations will play an increasingly significant role in critical dimensions error budget for lithography. Several suspected sources of LER in resist patterns include the reticle quality, the aerial image quality, and resist material properties.
An investigation of LER in resists [S. C. Palmateer, S. G. Cann, J. E. Curtin, S. P. Doran, L, M Erikersen, A. R. Forte, R. R. Kunz, T. M. Lyszczarz, and M. B. Stern, Proc. SPIE, 3333, 634 (1998)] has shown that surface roughness is low at both low and high exposure doses, but is at a maximum at intermediate doses. The affect of the aerial image on LER was described as arising from a similar intermediate dose transition region that would be typical of the edge of the aerial image. In this intermediate dose region, statistical fluctuations in polymer blocking level, composition, and molecular weight would lead to differences in resist development rate, leading to differences in the resulting linewidth. It was also noted that the LER depends on the aerial image contrast, given by the log of the image slope. As such, a low aerial image contrast results in a higher LER due to a larger transition region and a high aerial image contrast results in a lower LER due to a smaller transition region. The underlying resist material properties that lead to LER can be effected by imaging techniques that lead to either higher or lower aerial images. Thus, LER for different resists can only be compared when aerial image contrast is identical, e.g. identical imaging tool and test feature.
The LER of a resist arises from a linkage of the aerial image contrast and the resist material properties. The resist material properties that can be affected are statistical variations in the dispersion of photoacid in the film, statistical variations in the extent of acid catalyzed deprotection, and statistical variations on the solubility of the polymer chains. On average, the site density of photo-generated acid, polymer deprotection, and resist development are uniform. However, at the nanometer level, the local site density can fluctuate and lead to line edge roughness. Both nonuniform acid distribution and polymer deprotection will manifest itself in differential solubility of the polymer matrix in developer and these nonuniformities will be replicated as LER in the final resist image. Evidence for this was provided by Reynolds and Taylor [G. W. Reynolds and J. W. Taylor, J. Vac. Soc. Technol. B, 17, 334 (1999)] who found the largest contributor of LER to be the development process with variations in acid diffusion and shot noise having minimal effects on the overall LER.
The use of base additives in resists employed in the manufacturing of integrated circuits is established. Conventional wisdom in the field has been that the base additive should be present in the photoresist compound at a significantly lower concentration than the photoacid generator so as not to trap and neutralize the acid generated during exposure.
A need exists for photoresists that provide highly resolved fine line images, i.e., photoresists that have minimal to no contribution to LER or variations in line width. It is also desirable to formulate a photoresist that can be imaged at submicron and sub-half micron levels. Further, a need exists for a photoresist where variations in linewidth and line edge roughness are not effected by baking cycles used during manufacturing processes.